JP3428728B2 - Gray pixel halftone encoder - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to improvements in gray pixel encoding techniques. More specifically, the present invention relates to a gray pixel encoding technique used in combination with halftone image processing to produce a gray level halftone image. [0002] Xerographic printers use halftone processing to generate digital images from a document. This image is formed by using a plurality of pixels, which may be completely black, completely white, or shades of different grays. These are represented as explicit gray pixels. When viewed at a distance, the pixels mix to form an image. Conventional 2
In hex xerographic printers, the laser has only two laser intensity levels, ON (black) and OFF (white), because the halftone method used a binary device. Another method is U.S. Pat. No. 4,868,587 (by Locé et al.) That uses two gray levels for each pixel, a black level and a white level. The different gray levels are based on the intensity of the laser light. Thus, black has a higher light intensity than any gray level. This method is an improvement in using only black and white pixels. The disadvantage of this system is that the information cannot be easily converted to another printer due to the difficulty in controlling the gray levels. The quality of a binary xerographic printer is:
It is based on two important properties: the halftone frequency, which is the number of halftone cells per inch on a straight line, and the number of distinguishable gray steps. The typical number of halftone cells in good quality images is 100-2 per inch
00 cells. Normally, nine pixels are used in each halftone cell. The maximum number of gray steps is limited to one plus one pixel per halftone. Thus, a 3x3 halftone cell has about 10 output gray steps. In the prior art, a halftone cell was divided into a plurality of pixels that were converted either on or off to form a reflectivity change level. At some distance, the human eye is interpreted as a gray level. A reflectance of 5% can be detected. In a good quality printer, the number of distinguishable gray steps is about 100. Halftone images are commonly used in printed materials to copy continuous tone using a printer that is atomically binary. Conventional digital halftone processes have grown halftone dots by operating individual pixels in halftone cells. The pixels are typically at the resolution of the printer, or some small integer subdivision thereof. The resolution of the printer is divided into the number of spots used. For example, a 400 SPI printer has 1
It has a spot size of 1/400 of an inch. Pixels are used as data within the spot boundaries. SUMMARY OF THE INVENTION An advantage of the present invention is that, unlike conventional systems, halftoners and imagers are used.
r) and can be totally separated. Gray pixel encoding technology is used in combination with a halftone device and an imaging device to create a gray level halftone image. According to this method, the conventional disadvantage can be overcome. This is because halftone devices output encoded gray pixels that are decoded by the imaging device. The encoded gray pixels require less bandwidth than distinct gray pixels and can be decoded by the imaging device into an optical method for the particular printer in use. For example, the black process control changes the pixel decode table,
A print density that remains unchanged throughout the life of the printer can be assured. The imager produces small partial pixels within the spot boundaries. The generated sub-pixel is called a gray pixel. Gray pixels can evolve from zero width to full spot width and can start anywhere within the spot. These codes indicate the starting position of the gray pixel,
Describes the location of the gray pixel in the spot and the required gray pixel width. The imaging device uses the encoded information from the halftone device to generate a final halftone image. [0005] The list of encoded pixels generated by the halftone device may be stored, used later, and transmitted over a network to different imaging devices that make printouts. The pixel code is the same for all systems and the imaging device
In order to create a copy that is the original document itself, the generated halftone image is changed depending on the type of printer and its characteristics. The invention has been described in some detail herein, with particular reference to the embodiments shown, but it is to be understood that they are not intended to be limited to these embodiments. For example, the present invention can be used with any number of halftone cell configurations. Furthermore, the invention does not limit the printing device. Any image providing device that performs halftoning, such as a CRT display device, can use the halftone method described herein. The present invention uses a laser imaging system, commonly referred to as a raster output scanner (ROS), but with an LED.
It can be used for systems and similar systems. FIG. 1 is an example of an electronic printer 10. The printer 10 includes a xerographic processing section 12, a document scanning section 13, and an image printing section 14. The xerographic processing part 12
An endless belt-shaped light receiving device 20 is extended over the drive belt support roller 22 and the idler belt support roller 23. An electrostatic latent image device capable of displaying an image signal input is built on the light receiving device 20.
The belt support rollers 22, 23 are rotatably mounted at predetermined fixed positions by suitable means (not shown). The roller 23 is driven by a suitable drive motor (not shown) to move the light receiving device 20 in the direction indicated by the solid arrow. The light receiving device 20 may also be a light receiving device drum or other similar device. A corona charging device 30, commonly known as a corotron, is operably disposed on the charge station 31 adjacent to the light receiving device 20. The corotron 30 is connected to a suitable negative high voltage source (-HV) and places a uniform negative charge on the light receiving device 20 during preparation for image processing. The image printing portion 14 applies a variable pulse width image light beam 16 for scanning the entire light receiving device 20 to the exposure point 3.
4 The image beam 16 has a single self-modulating I.D.
R. It is extracted from the diode laser 25. A suitable lens 32 focuses the image beam 16 on the light receiving device 20. The electric charge on the light receiving device is such that the image beam 16 is
It is selectively dissipated when crossing zero. The electrostatic latent image formed on the light receiving device 20 corresponds to a document. The development subsystem 37, shown as a magnetic brush roll,
It is arranged downstream of the exposure point 34 in operative contact with the light receiving device 20. Toner, a relatively small dye substance, is loaded onto the magnetic brush roll of the development subsystem. The toner is attracted to the charge forming the electrostatic latent image by the electrostatic force on the charged light receiving device. The light receiving device 20 by the developing subsystem 37
Following development of the electrostatic latent image above, at transfer station 40, the developed image is transferred to a suitable copy sheet 4
1 (that is, paper). Transfer corotron 42 connected to a high voltage source (+ HV) to perform the transfer
Copies the image developed on the light receiving device 20 to the copy sheet 4
Attract one. Following the transfer, the developed image is fused by fusing. The remaining charge and / or developing material left on the light receiving device 30 is removed by a cleaning station 45 by an erase lamp 47 and a cleaning brush 46.
Is removed by In the document scanning section 13, image data is generated in the form of an electrical signal capable of indicating document reflectivity. The document 62 to be copied is placed on the transfer platen 60. Document positioning can be done either manually or by an automatic document handler (not shown). At least one linear device array 70 is mounted on a suitable carriage 64 supported to reciprocate back and forth below the platen 60.
Is attached. Array 70 may be in any suitable scanning array form, for example, a CCD. Carriage 64 is driven by a suitable reversible driver, such as a stepper motor (not shown). Lens 7
2 is focused on the array 70 on the line of the document 62.
A suitable lamp 74 illuminates the document line being scanned by the array 70. Output signals from array 70 are transmitted to processor 80. Alternatively, the modem 8 may be used to receive signals from another source, ie, a computer and thereby bypass the document scanning portion 13.
2 can be used. FIG. 2 shows a block diagram of a conventional processor 80 that generates a halftone image 90. The continuous tone image 82 is a continuous tone image displaying gray pixels.
For example, an 8-bit word is used for each pixel to represent 256 levels of gray. Halftone design 84 is a binary representation of the image. The halftone design 84 includes a plurality of rules that dictate a fill-in command and a screen design. The fill-in instruction determines which pixels on the halftone cell are turned on first, second, third, and so on. For example, an N × M array has, for each pixel,
Command whether it is on or off. Screen design is the direction of the pixels on the screen. For example, the pixels could be aligned at 0 ° and the rows and columns could form a grid of pixels. Pixels can be aligned at 45 ° to form a diagonal grid. For color printing, pixels are 0 °, 45 °, ±
There are four angles that are aligned at 15 °. Another feature of the screen design is the switching instructions at the pixels. In a cluster dot design, when a pixel starts the design, neighboring pixels are switched on in a row, column, and spiral configuration. In a scattered dot design, adjacent pixels do not start to turn on,
The pixel is switched on. [0010] Continuous tone image 82 and halftone design 8
4 is input to the halftone device 86. Halftone device 86 generates the bits that form the distinct gray pixels. These bits indicate the binarized image that is input to the imaging device 88. The imaging device forms a halftone image 90 that operates the laser 25 of FIG. FIG. 3 shows a block diagram of the processor of the present invention.
The continuous tone image 102 and the halftone design 104 are input to a halftone device 106. Pixel code 10
8 performs a predetermined decoding operation so as to bring a pixel into a specific state. For example, if a pixel can be displayed in 11 different states, the characteristics of each state will not change.
These pixel codes 108 are input to the halftone 106. Encoded gray pixel 110
Is generated from the three signals by the halftone device 106. These encoded gray pixels 1
10 requires less bandwidth (ie, fewer bits) than conventional well-defined gray pixels. For example, this distinct gray pixel may consist of 8 bits, but due to the standardized use of the pixel code 108, the encode gray pixel 110 may be only 4 bits. These encoded gray pixels 110
Is then delivered to the imaging device 112 for immediate use. Alternatively, the encoded gray pixel 110
Can also be stored for later use. In addition, the encoded gray pixels 110 can be decoded by the imaging device 112 into an optical method depending on the mounting configuration of the printer. Imaging device 112 has the darkness specified for that particular printer. For example, each printer shows how long it has been used,
It can be changed depending on the age of the printer, the alignment of the laser and the lens, and the like. Standard encoded gray pixel 110
, The imaging device 112 can be compensated for these variable characteristics. Another advantage of the present invention is that the encoded gray pixels 110 can be used on different imaging devices 112 and that they have the same result (this is particularly useful in network systems with more than one printer). is there. The encoding operation of the present invention specifies the darkness of gray pixels. This includes the pixel density and the location within the pixel where the gray operation occurs. Other pixel attributes, such as color separation, may also be encoded. This allows the imaging device to treat pixels colored differently in different ways. Therefore, according to the encoding operation, the halftone cell can be defined independently of the printing operation. This is because the imaging device creates a special darkness for that particular printer. The encoding technique is described below. Gray pixels will be generated by image processing hardware using a look-up table (LUT). This look-up table allows a single variable pulse width to be located anywhere within a pixel boundary. The LUT of the preferred embodiment includes 256 elements, but any number of elements can be used.
The LUT encodes both the location and width of the gray pixel. Therefore, some bits of the gray pixel output from the halftone device are used to encode the position, and some of those bits are used to encode the pulse width. For example, a pixel may have two intermediate gray levels plus white and black. Pixels are considered to have three gray levels because white is not included as a gray level. A single development rule may be used, which e.g. develops from the center. Thus, a pixel can use two bits to encode all four levels of the pixel. If two development rules are used, that is, if one develops from either the right or left edge of a pixel, one bit is needed to encode which development rule to use. Therefore, 2-bit pixels are not appropriate. By letting the gray pixels enforce the development rules,
Because the location information is included, the encoded halftone pixel bits can be used to represent more gray levels. Referring to FIG. 4, a gray pixel using only one development rule is shown. The rule is to meet from the center. Thus, there are five unique states for this one rule. This first example restricts the gray pixel pulse to start at the center of the pixel and fill it all. If one rule is indicated, no information needs to be encoded in the pixel. The number of states required to encode a gray level is as follows: S = G + 1 (1) where S is the number of states required to encode the gray level, G is the number of gray levels allowed for that pixel, and “1” is the white pixel For the state. In FIG. 5, two rules are used. One rule allows pixels to be filled from the right, and the second rule allows pixels to be filled from the left. Therefore, there are eight unique states that use two rules. The unique number of states can be determined by the following equation: S = G + (G-1) + 1 = 2G (2) where S is the number of states and G is the number of gray levels allowed for that pixel. In FIG. 6, there are three rules that allow ten unique states. These rules are development from the right, development from the left, and development from the center. The unique number of states is
It is determined by the following equation. S = G + (G−1) + (G−2) + 1 = 3G−2 (3) As in the equations (1) and (2), S and G represent the same function. In general, a unique number of states is required for any given number of rules, as determined by the following equation: ## EQU1 ## Where S is the number of states required to encode a gray level, G is the number of gray levels allowed for that pixel, R is the number of rules,
“1” indicates the state of the white pixel. To determine the number of states needed to encode all possible pulse widths and position states, the following equation is used. ## EQU2 ## By finding the maximum number of fill-in rules using this formula, any pixel in the cell can have any allowed pulse. FIG. 7 shows four gray levels (G =) for forming 11 possible states.
4) exists. The number of states given by Equation 5 is
Suitable for encoding any development rules for pixels. This is called the "N rule" case. For example, a line screen is made by using only one development rule that commands development from its center outward. Older halftone designs require three rules: development from the left, development from the right, and development from the center. The number of bits required to encode all states is simple: B = log ₂ (S) (6) Where B is the number of bits and S is the number of states. By solving G using Eq. 5 instead of S in Eq. 6, the maximum number of gray levels per pixel for a given pixel depth (bits / pixel), and any special Calculate the number of rules required for a halftone cell design. Referring to the table shown in FIG. 8, this calculation is from one bit per pixel to
Done for 8 bits and for 1, 2, 3, and N development rules. For example, if there is one bit per pixel and one development rule is used, only one gray level is encoded per pixel,
That is, it is black. Thus, one pixel can only have either black or white states. If there are two bits per pixel and only one development rule, then there are three gray levels encoded per pixel: black, white, and two gray levels. Referring to FIG.
A halftone cell 200 consisting of two pixels 202-218 is shown. First, halftone cell 200
Starts with half a gray cell at pixel 210 and the remaining pixels are white. Since the encoded pixel has a code in such a state that 1/4 gray is required, the pixel code determines in the imaging device that a center going left or right is needed. The encoding operation can extend the pixel fill operation to be half-gray. The encoding operation need not fill the entire pixel 210, but will begin to fill neighboring pixels 212 with a quarter gray, starting from the left and moving toward the center of pixel 212. Similarly, the upper and lower pixels can be used to increase the gray component of halftone cell 200. This method is called cluster dot design and can be used with raster output scanners. In a scattered dot design, the pixels 204 can be encoded to partially fill from the right, and the pixels 212 can be encoded to partially fill from the left. This result is shown in FIG.
0 is a halftone cell. One advantage of the present invention is that the halftone 106 and the imaging device 112 can be totally separated. In the prior art, they were always linked. In the present invention, the halftone device 106 can generate a list of encoded pixels 110 by using a pixel code 108. This encoded pixel 110 can be stored for later use and transmitted over a network to a different imaging device that performs the printout. The pixel code 108 is system invariant, and any image device will be able to modify its painting depending on the type of printer and its special characteristics to repeatedly produce the original image copy.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an example of an electronic printer. FIG. 2 is a block diagram of a printer that generates a halftone image using a conventional method. FIG. 3 is a block diagram of a printer that generates a halftone image using the method of the present invention. FIG. 4 illustrates a gray pixel generated by using one development rule. FIG. 5 shows a gray pixel generated by using two development rules. FIG. 6 shows a gray pixel generated by using three development rules. FIG. 7 shows possible pulse widths and position states for one pixel with four gray levels. FIG. 8 is a table showing the number of gray levels that can be encoded in one pixel for different development rules. FIG. 9 is a halftone cell with some encoded gray pixels in a cluster dot design. FIG. 10 is a halftone cell with some encoded gray pixels in a scattered dot design.

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Claims (1)

(57) [Claim 1] In a method for generating a halftone image from an original image, a continuous tone image and a halftone design corresponding to the original image are provided.
And inputting the halftone design
In a halftone cell consisting of multiple pixels
A rule that determines the order in which each pixel is turned on,
Screen that determines pixel orientation and screen shape
Consisting of rules for determining the down design, and the input stage, and said continuous tone image, said halftone design, Pic
Generating a gray pixel having an encoded pixel position and width based on the pixel code used in decoding the state of the cell; and determining the encoded gray pixel based on the encoded gray pixel and the pixel code. And generating a halftone image.